The invention relates to an optical arrangement, for example a projection lens for microlithography, in particular for EUV lithography, an EUV lithography apparatus comprising such a projection lens, and a method for configuring an optical arrangement.
Reflective optical elements (mirrors) having a dielectric coating are used in optical arrangements for EUV lithography. Such mirrors have a reflectivity of typically less than 70% for the impinging EUV radiation, such that a considerable proportion of radiation is absorbed by the mirrors and converted into heat. Materials used as substrates for mirrors in EUV lithography are permitted to have only a very low coefficient of thermal expansion (CTE) in the range of the operating temperatures used there, on account of the extremely stringent requirements in respect of geometrical tolerances and stability that have to be imposed on the mirror surfaces in particular in projection lenses used there. In order to achieve this, the substrate materials used in EUV lithography typically have two constituents, the coefficients of thermal expansion of which have a mutually opposite dependence on temperature, such that the coefficients of thermal expansion almost completely compensate for one another at the temperatures that occur at the mirrors during the operation of the EUV lithography apparatus.
A first group of materials that satisfies the stringent requirements with regard to the CTE for EUV applications is doped silicate glasses, e.g. silicate or quartz glass doped with titanium dioxide, typically having a silicate glass proportion of more than 80%. One such silicate glass that is commercially available is sold by Corning Inc. under the trade name ULE® (Ultra Low Expansion glass). It goes without saying that TiO2-doped quartz glass can, if appropriate, also be doped with further materials, e.g. with materials which reduce the viscosity of the glass, as is explained e.g. in US 2008/0004169 A1, wherein alkali metals are used, inter alia, in order to reduce the effects of striae in the glass material.
A second group of materials suitable as substrates for EUV mirrors is glass ceramics, in which the ratio of the crystal phase to the glass phase is set such that the coefficients of thermal expansion of the different phases almost cancel one another out. Such glass ceramics are offered e.g. under the trade name Zerodur® by Schott A G or under the trade is name Clearceram® by Ohara Inc.
The dependence of the thermal expansion (change in length) of the above-described materials on temperature is approximately parabolic in the relevant temperature range, that is to say that there is an extremum of the thermal expansion at a specific temperature. The derivative of the thermal expansion of zero expansion materials with respect to temperature (i.e. the coefficient of thermal expansion) is approximately linearly dependent on temperature in this range and changes sign at the temperature at which the thermal expansion is an extremum, for which reason this temperature is designated as the zero crossing temperature (ZCT). Consequently, the thermal expansion is minimal only for the case where the operating or working temperature of the substrate coincides with the zero crossing temperature.
The zero crossing temperature can be set within certain limits during the production of the substrate materials or the blanks, for example by choosing suitable parameters during heat treatment or, in the case of TiO2-doped quartz glass, by setting the titanium dioxide proportion used during the production of the quartz glass. The zero crossing temperature in the substrate and in particular in the vicinity of the optical surface is in this case typically set to be as homogeneous as possible.
However, the radiation intensity or irradiance impinging on the optical surfaces during the operation of the mirrors is not homogeneous and varies in a location-dependent manner, which means that the resulting temperature distribution at the optical surface is also inhomogeneous. Consequently, the condition that the operating temperature corresponds to the zero crossing temperature cannot be fulfilled at the entire surface, such that the latter in the case of operation is not totally insensitive to temperature and thus free of deformation. Although the coefficient of thermal expansion is still small in the case of small deviations of the operating temperature from the zero crossing temperature, it increases further as the temperature difference with respect to the zero crossing temperature increases, which can lead to deformations of the reflective surface on account of the locally different linear expansion and to deformation-governed wavefront aberrations.